Beam width defining structure for linear accelerator radiotherapy devices



Nov. 10, 1970 RESNCK BEAM WIDTH DEFINING STRUCTURE FOR LINEAR ACCELERATOR RADIOTHERAPY DEVICES Flled Nov 24 1967 2 Sheets-$heet 1 /j/J E IN "FNTOR.

LARRY RESbil ihxW ATTORNEY Nov. 10, 1970 L. RESNICK 3,539,313

BEAM WIDTH DEFINING STRUCTURE FOR LINEAR 1 ACCELERATOR RADIOTHERAPY DEVICES Filed Nov. 24, 1967 2 Sheets-Sheet 2 FIG. 2

INVFNTUR.

LARRY RESNICK \flM F ATTORNEY United States Patent O 3,539,813 BEAM WIDTH DEFINING STRUCTURE FOR LINEAR ACCELERATOR RADIOTHERAPY DEVICES Larry Resnick, Palo Alto, Calif, assignor to Varian Associates, Palo Alto, Calif, a corporation of California Filed Nov. 24, 1967, Ser. No. 685,533 Int. Cl. G03b 41/16 US. Cl. 250-105 2 Claims ABSTRACT OF THE DISCLOSURE A pair of heavy metal jaws have facing surfaces which define the width of an X-ray beam passing between the jaws. Each jaw is supported on (1) a threaded shaft perpendicular to the beam axis and (2) a shaft which forms an angle with the threaded shaft. The angle and the positions of attachment between jaws and shafts are selected so that the facing surface on each jaw is maintained substantially parallel to the beam edge as the jaws are moved, thus reducing stray radiation at the beam edges.

BACKGROUND OF THE INVENTION The present invention relates in general to a novel X- ray or charged particle beam width defining apparatus for use in high energy X-ray therapy, diagnostic radiography, electron therapy and other irradiation techniques. In particular, the invention relates to means for eliminating the stray or penumbral radiation which exists at the beam edges in some prior art X-ray apparatus.

Modern methods of treating cancer and other related diseases demand high intensity levels of radiation for deep X-ray therapy applications. Therefore, high energy devices in the range of 4 to 8 mev. are employed to obtain the desired intensity and quality of radiation.

In order to effectively irradiate a deep seated tumor, for example, techniques such as arc, multiple port and patient rotation therapy have evolved. Each of these techniques, when used in conjunction with high energy X- radiation, requires that the cross sectional dimensions of the X-ray beam be accurately controllable. Furthermore, the increasing awareness of the danger of stray radiation has brought about requirements that the beam edges be sharply defined in order to reduce the amount of radi ation falling outside the desired beam limits.

For the purpose of defining the beam edges prior art high energy X-ray machines use large and heavy movable jaws disposed in pairs on opposite sides of the X-ray beam axis. Such a machine is described in US. Pat. 3,322,- 950, issued May 30, 1967, and assigned to the assignee of the present invention. The jaws are typically made of lead and dimensioned so that the thickness of the lead in the direction of X-radiation is sufficient to reduce the intensity of X-rays passing through the block by a fac tor of Motion of the jaws is generally in a direction toward or away from the X-ray beam axis in order to define beam width.

To ensure that the beam of X-rays is sharply defined near the beam edges or, alternatively stated, that the radiation intensity falls off abruptly at the beam edge, it is necessary that the beam defining inner surfaces of the jaws be parallel to the beam edges. Stated another way, the beam defining surfaces must lie in planes intersecting the X-ray target. When the beam edge defining surfaces lie in planes intersecting the target, X-rays passing near the beam edge either miss the jaw completely thus forming part of the desired beam or else intersect the jaw at the upstream edge thereof and are attenuated by the full thickness of the jaw. In this way, substantially all of the X-rays striking the jaws must pass through the ice entire thickness of the jaw and Will thus be attenuated by a factor of In order to satisfy the above condition throughout the range of movement of the jaws, the apparatus of the above mentioned patent used curved tracks comprising portions of a circle centered on the target. Rollers mounted on the jaws rode on the track and caused the beam-defining surfaces to be parallel to the beam edges throughout the range of jaw motion.

Although this prior system worked well, it was complex and expensive. Double tracks were required in order to support the jaws, since the X-ray head moved 360 in a vertical plane for arc or multi-port therapy. The tracks and rollers had to be precisely made and the tolerances carefully adjusted in order to prevent undesirable shifting or rocking of the jaws when the head was rotated.

SUMMARY OF THE INVENTION According to the present invention a novel linkage system is used to support the jaws and to constrain the motion thereof so that the beam-edge-defining facing surfaces lie in planes intersecting the target. A single horizontal shaft extends between the jaws and has two oppositely threaded portions thereon which act as lead screws to simultaneously move both jaws when the shaft is rotated. A second shaft on each jaw is angled toward the horizontal shaft. Each jaw of the pair is mounted on the horizontal shaft by a pivot which engages the threads and also permits pivotal motion of the jaw about an axis intersecting the shaft. A second pivot on each jaw is carried on the second shaft and permits pivotal motion of the jaw about an axis intersecting that shaft and linear sliding motion along the shaft.

A second set of jaws, rotated from the first set, is disposed about the beam axis adjacent the first set of jaws and is operated in a similar fashion.

By suitable arrangement of the angles between the shafts and the locations of the pivots with respect to the jaws, the facing surfaces of the jaws can be made to move substantially in arcs of a circle centered upon the target.

The principal object of the present invention is to provide improved apparatus to adjustably define the width of a beam of radiation while substantially reducing stray or penumbral radiation at the beam edges.

Another feature of the present invention is to provide a novel linkage system, comprising two rigid non-parallel shafts, which moves a heavy metal jaw disposed adjacent the beam axis of an X-ray beam toward or away from the beam axis while maintaining the beam-limiting surface of the jaw parallel to the edge of the beam.

Another feature of the present invention is to provide a novel linkage system for supporting a pair of heavy metal jaws and simultaneously moving the jaws toward or away from an X-ray beam passing between the jaws while maintaining the facing surfaces of the jaws parallel to the edges of the beam, wherein the linkage includes a threaded shaft on either side of each jaw to move the jaws in response to rotation of the shafts and means to couple the rotation of the shafts.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view of a radiotherapy linear accelerator utilizing the novel beam defining apparatus of the present invention;

FIG. 2 is an enlarged partially sectional view of a portion of the structure of FIG. 1 delineated by the line 22 in FIG. .1;

FIG. 3 is an enlarged sectional view taken along lines 3-3 of FIG. 2 showing details of the pivots used to support the beam defining jaws.

3 DESCRIPTION OF THE PREFERRED EMBODIMENT Referring in more detail to FIG. 1, there is shown a side view of a radiotherapy linear accelerator 1, having a main accelerator section 15, an electron gun 2 and colinear termination such as shown and described in US. Pat. 3,264,515 assigned to the same assignee as the present invention, beam bending end section 3, rotatable beam defining head 4 and counterweighted modulator portion 5. The main accelerator section is designed to rotate 360 about horizontal axis 6, which is located, by way of example, approximately 117 cm. above the floor level. The main accelerator section can be fixed in any radial po sition about horizontal axis 6 for therapy treatment of a patient as shown. The rotatable beam defining head 4 is designed to rotate 360 about central beam axis 12 and can easily be moved into any desired position with respect to the patient such as the positions shown.

A special treatment couch 8 with detachable wheel assemblies is provided which is clamped or otherwise suitably attached to float table 10 which is supported by post 11 which is arranged for dual speed vertical movement and which in turn is mounted on a rotatable floor level platform 13 designed for circular movement about the central beam axis 12. Couch 8 has a thin Mylar or other suitable supporting material 9 attached thereto whereby the patient can be irradiated from a posterior position as shown wtihout danger of ionization build-up and severe skin reactions. The float table 10 is provided with a means for translational movement of the patient in the horizontal lane. p It is readily seen that the spatial relationships between the accelerator horizontal rotational axis 6, the central axis 12 of the X-ray beam and the vertical rotational axis 12 of the patient support assembly result in the intersection of the above three axes at a fixed point, the isocenter 7.

The above indicated relationships facilitate patient setup procedures and provide a radiotherapy linear accelerator system which is capable of multiple port, patient rotation and arc therapy treatments in a manner which is extremely advantageous from a clinical standpoint as well as from time, space, and monetary considerations.

FIG. 2 illustrates in detail the novel beam width defining apparatus of the present invention. An X-ray target 16 is bombarded by a downwardly directed beam of electrons (not shown) thereby producing a downwardly directed X-ray beam. A tungsten collimator 18 has a conically shaped central aperture through which the beam passes to strike a field flattener 19, and an ionization cham ber 20. Chamber 20 may be used to measure total radiation intensity while field flattener 19 promotes a uniform beam intensity throughout the beam cross section.

A 360 rotatable circular mounting plate 22 is supported from the remainder of accelerator 1 by a ball bearing 24. Side support members 26 are fastened to plate 22 and serve to support the remainder of the structure of FIG. 2. A bowl shaped cover 27 is supported from support members 26 by suitable means (not shown) and rotates therewith. A horizontal support shaft 28 is supported by a bracket 30 in ball bearings 32, and from a front cover plate 34 (partially broken away) by a central bearing 36. Bracket 30 and plate 34 are rigidly attached to side support members 26.

Shaft 28 has two oppositely threaded portions 35 which act as lead screws to move a pair of heavy metal lower jaws 38 when shaft 28 is rotated. As seen in FIGS. 2 and 3, jaws 38 are supported on shaft 28 by lower pivots 40 comprising trunnion supports 42 and nuts 44 which engage the threads of threaded portions 36 and are held within trunnion supports 42 by end flanges 46. laws 38 have facing surfaces 45 which serve to define beam width.

Trunnion supports 42 comprise a lower shell 48 and an upper shell 49 fastesned to lower shell 48 by screws 51. Lower shell 48 has a pivot shaft 50 extending through a 4 washer 52 and into an adapter 54 which is located together with a ball bearing 56 in jaw 38. When screws 51 are tightened nut 44 is prevented from turning relative to the trunnions.

Angled support shafts 58 are held on their outer ends by ring anchors 60 which are fastened by arms 62 to side support members 26. A pair of lock nuts 64 provide for height adjustment of the outer ends of shafts 58 to thereby change the angle of shafts 58. The inner ends of shafts 58 are supported in similar fashion by ring anchors 66 passing through front cover plate 34.

As seen in FIGS. 2 and 3 jaws 38 are supported upon shafts 58 by upper pivots 68 which comprise a trunnion shell 70 which encloses a bushing 72, and a pivot shaft extending through a washer, adapter and ball bearing in similar fashion to lower pivots 40.

A second set of horizontal and angled shafts, upper and lower pivots, etc. (not shown) is disposed on the opposite side of jaws 38.

Upper jaws 74 identical to lower jaws 38, together with horizontal shafts 76 and angled shafts, upper and lower pivots, etc. (not shown) are positioned rotated 90 about the beam axis from jaws 38. Upper and lower drive mechanisms, used to provide adjustment of either set of jaws, comprise drive sprockets 78, idler sprockets 80 and drive chains 82. An adjustment wheel 84, shown only on the lower drive mechanism but included also on upper drive mechanism, is provided on one of idler sprockets 80 whereby the horizontal shafts may be turned, causing synchronous motion of the corresponding jaws by driving nuts 44 along shaft 28. When nuts 44 are driven along screw shaft 28, the resulting movement of the jaws will cause pivots 68 to slide along shafts 58.

In operation the motion of lower jaws 38 toward or away from the beam axis is accompanied by simultaneous pivotal movement of the jaws about the pivot axes 86 and 88 of the upper and lower pivots 68 and 40 respectively. When jaws 38 are moved away from the beam axis they are thus simultaneously rotated in a direction such that the lower edges of jaws 38 are tilted away from the beam axls.

By suitable selection of the angle between horizontal shaft 28 and angled shafts 58, and the distance between pivot axes 86 and 88, the facing surfaces 45 can be maintained substantially parallel to the beam edges throughout the range of adjustment of beam widths. In other Words, the faces 45 stay in planes which substantially intersect the center of source 16.

Although the invention has been described with some particularity in reference to a specific example, it will be obvious to the skilled worker in the art that many modifications could be made without departing from the scope of the invention.

Accordingly, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. Apparatus for defining the width of a beam of radiation about a central axis comprising first and second jaws of a radiation absorbing material having facing surfaces thereon for defining the edges of said beam, both said first and second jaws being movable toward and away from said central axis to vary the width of said beam within a preselected range; motion defining means connected to said jaws to constrain the motion thereof so that the facing surfaces thereof are maintained substantially parallel to the beam edges defined thereby at any location of said jaws within said preselected range, said motion defining means comprising a first support shaft extending between said jaws and being connected to each of said jaws by first pivot means permitting pivoting of said jaws about axes intersecting said first shaft, a pair of second support shafts, each of which is respectively associated with a corresponding one of said jaws and each of which is connected to its associated jaw by second pivot means permitting pivotal motion of said jaws about an axis intersecting its associated second shaft and permitting linear motion of said jaws along their corresponding second shafts, said first shaft being threadably received within said first pivot means associated with each of said jaws by threads of opposite pitch whereby upon rotation of said first shaft said first pivot means and thereby said jaws are moved simultaneously toward or away from said central axis and whereby said first shaft acts not only to constrain the motion of said jaws but also to impart motion thereto; and means for providing rotation of said first shaft.

2. Apparatus for defining the width of a beam of radiation according to claim 1 wherein said motion-defining means is at one side of said first and second jaws and a second motion-defining means identical to the first motiondefining means is on the oppositeside of said jaws, and rotation coupling means are provided coupling the first shafts of said motion defining means to rotate such shafts synchronously.

References Cited UNITED STATES PATENTS 4/ 1963 Varga et a1 250-105 9/1964 Wilson 250-105 

